Type A smectic liquid crystal having a positive dielectric anisotropy

ABSTRACT

A family of liquid crystals having a type A smectic mesophase with a positive dielectric anisotropy, which are particularly useful in the case of the thermo-optical effect. 
     The substances of the family are in accordance with the general formula: ##STR1## in which R represents an organic group of the alkyl or alkoxy type containing 1 to 15 carbon atoms and in which X and Y designate either the bromine or the nitrile radical.

The present invention relates to a liquid crystal, whose general formulacorresponds to a family of substances only differing from one another byan alkyl or alkoxy group and by a bromo or cyano radical. In certaintemperature ranges, these substances have a type A smectic mesophasewith a positive dielectric anisotropy. Mixtures of these substances withone another and with other liquid crystals are included within the scopeof the invention. They can be used in visual displays where theadvantage of the thermo-optical effect is utilized, with or withoutassistance of an electrical field.

The liquid crystal according to the invention is in accordance with thegeneral formula: ##STR2## in which R represents an organic group of thealkyl or alkoxy type having 1 to 15 carbon atoms and in which X and Ydesignate either a bromo or cyano radical.

These substances are designated by:

(1) p-bromodiphenyl-4-(alkyl or alkoxy)-3-bromobenzoate, if X=Y=Br

(2) p-cyanodiphenyl-4-(alkyl or alkoxy)-3-bromobenzoate, if X=Br andY=CN

(3) p-cyanodiphenyl-4-(alkyl or alkoxy)-3-cyanobenzoate, if X=Y=CN.

Hereinafter, the mechanism of the general production process of theinvention is given, followed by a number of examples of operatingprocedures.

GENERAL PRODUCTION PROCESS

Firstly, 4-hydroxy-4'-bromodiphenyl and, as applicable,4-alkoxy-3-bromobenzoic acid or 4-alkyl-3-bromobenzoic acid aresynthesized to obtain brominated benzoates. The bromine in thebrominated compounds is replaced by nitrile to obtain CN-containingcompounds.

1. Synthesis of 4-hydroxy-4'-bromodiphenyl

The starting substance is diphenyl acetate with which bromine is reactedat 30° C. in the presence of iodine as the catalyst. The reactiondiagram is as follows: ##STR3##

The product obtained is saponified by potassium hydroxide in anethanolic medium and then hydrolyzed by hydrochloric acid in accordancewith the following diagram: ##STR4##

In the case of products X=Br and Y=CN it is necessary to synthesize4-hydroxy-4'-cyanodiphenyl. It is obtained from the aforementionedproduct by reacting cuprous cyanide in dimethyl formamide:(DMF) ##STR5##

2. Synthesis of 4-(alkyl or alkoxy)-3-bromobenzoic acid

By bromination, the corresponding 4-substituted benzoic acid isobtained: ##STR6## For R=C_(n) H_(2n+1), reaction takes place in anaqueous solution of nitric and acetic acid in the presence of silvernitrate at 25° C.

For R=C_(n) H_(2n+1) O, the reaction takes place in an aqueous medium at55° C.

3. Synthesis of the chloride of the aforementioned acid

The diagram is as follows: ##STR7##

4. Synthesis of diphenylbenzoates

Three cases are to be envisaged:

(1) Case in which X=Y=Br:

4-hydroxy-4'-bromodiphenyl is esterified with the selected 3-bromo acidchloride at ambient temperature in a medium constituted by pyridine:##STR8##

(2) Case where X=Br and Y=CN:

Esterification takes place between the acid chloride and the4-hydroxy-4'-cyanodiphenyl in the same medium (pyridine): ##STR9##

(3) Case where X=Y=CN:

The two Br groups are substituted by CN on products of type (1) in amedium containing cuprous cyanide and dimethyl formamide:

EXAMPLES OF OPERATING PROCEDURES (R=C₇ H₁₅, X=Br, Y=CN) (a) Synthesis of4-n-heptyl-3-bromobenzoic acid

8.8 g (0.04 mole) of 4-n-heptyl-3-bromobenzoic acid and 8 g (0.05 mole)of bromine are introduced, accompanied by stirring, into a mixture of 20ml of soft water, 27 ml of nitric acid and 120 ml of glacial acetic acidheated to 25° C. A solution of 6.8 g (0.04 mole) of silver nitrate in 20ml of soft water are then added over a period of 30 minutes, whilstmaintaining the temperature at 25° C. Stirring is continued for 31/2hours. The solution is then poured into 200 ml of ice water and stirredfor 30 minutes. The product in suspension is filtered and washed withsoft water up to neutrality. The acid is then separated from the silverbromide formed by dissolving in ethanol. The ethanolic solution isevaporated. 10.5 g of crude product is obtained, which is recrystallizedin 30 cc of hexane. 9.1 g of synthesized acid is recovered. The meltingpoint observed in 80° C.

(b) Synthesis of 4-hydroxy-4'-cyanodiphenyl

1 mole or 249 g of 4-hydroxy-4'-bromodiphenyl 1.25 mole or 113 g ofcuprous cyanide and 1 liter of dimethyl formamide are introduced into areactor. Refluxing, accompanied by stirring, takes place for 6 hours.The reaction mass is then poured into a reactor containing ethylenediamine and water. Extraction takes place with chloroform (2×1 liter),then the organic phase is washed to neutrality. After drying andevaporating the chloroform, 100 g of crude product are obtained. 47 g ofproduct are obtained after chromatography on silica with benzene as theeluent. The synthesis yield is approximately 24%.

(c) Synthesis of 4'-cyanodiphenyl-4-n-heptyl-3-bromobenzoate

0.5 g (2.5.10⁻³ mole) of 4-hydroxy-4'-cyanodiphenyl are dissolved in 5ml of pyridine and 0.8 g (2.5.10⁻³ mole) of 4-heptyl-3-bromobenzoylchloride is added thereto. The mixture is stirred for 3 days at ambienttemperature and then poured into a mixture of 20 g of ice and 2 cc ofconcentrated sulphuric acid. After stirring for 4 hours, the product isextracted with benzene, washed in soft water to neutrality and dried. Itis then purified by silica column chromatography by eluting with ahexane: benzene (50:50) mixture. The product obtained is dissolved inethanol and treated with vegetable black, followed by recrystallizationin 15 cc of ethanol. 300 mg of pure product are collected, giving ayield of 25%. The product has the following characteristics: K 89.5S_(A) 129 N 151 I (temperatures in °C.) with the following conventions:

K crystalline phase

S_(A) smectic phase A

N nematic phase

I isotropic liquid.

Table I summarises the formulas and temperature ranges of a number ofexamples of substances according to the invention.

                                      TABLE I                                     __________________________________________________________________________                                      Temperature ranges                          Example                                                                            Formula                      and heat of fusion.                         __________________________________________________________________________          ##STR11##                   K90.5S.sub.A 152.5 I and 4.8 Kcal/mole      2                                                                                   ##STR12##                   K121 S.sub.A 172 N 176 I                    3                                                                                   ##STR13##                   K89.5 S.sub.A 129 N151 I and 6.5                                              Kcal/mole                                   4                                                                                   ##STR14##                   K131 [S.sub.A ] 94 [N] 117 I and 9.0                                          Kcal/mole                                   5                                                                                   ##STR15##                   K125 [N] 152 I                              __________________________________________________________________________

The transition points between the phases are in degrees Celsius. Thephases in square brackets are of the metastable type.

The dielectric properties of the substances of examples 1 and 3 weredetermined on the basis of measurements carried out on the mixture ofeach of the products with a nematic liquid crystal "F", p-pentyl phenolp-methoxybenzoate. "F" is a nematic of 29° to 43° C. The composition ofthe mixture is 90% of "F" and 10% of the compound according to theinvention in molar fractions. The mixture is itself nematic. Themeasurements are performed at 22° C., the magnetic orientation field is10⁴ oersteds and the frequency of the electrical measuring field is 10kHz. The substances of examples 1 and 3 are designated by letters A andC, giving the following results table:

                  TABLE II                                                        ______________________________________                                        Case of:     ε.sub.∥                                                                  ε.sub.⊥                                                                 ε.sub.a                               ______________________________________                                        Pure F       5.7         5.6    +0.1                                          Mixture of A + F                                                                           5.1         4.8    +0.3                                          Mixture of C + F                                                                           6.8         5.1    +1.7                                          ______________________________________                                    

The pure products A and C therefore have a positive dielectricanisotropy. On the basis of a linear variation law, ε_(a) as a functionof the concentration the following values are obtained: ε_(a) (A)=2,ε_(a) (C)=16. Thus, A is slightly positive and C highly positive.

Eutectic mixtures of compounds according to the invention with othertype A smectic liquid crystals can be used in particular withp-cyanodiphenyls having the smectic phase A.

The following table gives examples of mixtures:

                  TABLE III                                                       ______________________________________                                        Formulas of the ingredients and                                                                       Molar fraction of                                     temperature range       each ingredient.                                      ______________________________________                                         ##STR16##              0.82                                                  and                                                                            ##STR17##              0.18                                                  Range: K 15.5 S.sub.A 53 N 57.5 I                                              ##STR18##              0.89                                                  and                                                                            ##STR19##              0.11                                                  Range: K 18 S.sub.A 44 N 53 I                                                  ##STR20##              0.64                                                   ##STR21##              0.22                                                  and                                                                            ##STR22##              0.15                                                  Range: K 9.0 S.sub.A 54 N 57 I                                                 ##STR23##              0.54                                                   ##STR24##              0.18                                                   ##STR25##              0.13                                                  and                                                                            ##STR26##              0.15                                                  Range: K 5 S.sub.A 57 N 63 I                                                   ##STR27##              0.58                                                   ##STR28##              0.19                                                   ##STR29##              0.14                                                  and                                                                            ##STR30##              0.09                                                  Range: K 7 S.sub.A 63 N 69 I                                                  ______________________________________                                    

These mixtures can be directly used in thermo-optical devices such asmatrix screens or laser addressing screens. They all have the requisitecharacteristics of a wide smectic A range including the ambient, anarrow nematic range, a positive dielectric anisotropy, a good diffusingpower and therefore a good constant and a low operating power.

What is claimed is:
 1. A liquid crystal, having type A smectic phase, ofthe formula ##STR31## wherein R represents C_(n) H_(2n+1) or C_(n)H_(2n+1) O with 1≦n≦15; andX represents Br or CN.
 2. The liquid crystalaccording to claim 1, wherein:R is C₇ H₁₅ and X is Br.
 3. The liquidcrystal according to claim 1, wherein:R is C₈ H₁₇ O and X is Br.
 4. Theliquid crystal according to claim 1, wherein:R is C₇ H₁₅ and X is CN. 5.The liquid crystal according to claim 1, wherein:R is C₈ H₁₇ O and X isCN.
 6. A liquid crystal composition, having type A smectic phase,comprising a first liquid crystal according to claim 1 and at least oneadditional type A smectic liquid crystal.
 7. The liquid crystalcomposition according to claim 6, wherein said first liquid crystal is##STR32## and said additional liquid crystal is ##STR33## and whereinthe molar proportions of said first liquid crystal and said additionalliquid crystal are 0.18 and 0.82, respectively.
 8. The liquid crystalcomposition according to claim 6, wherein said first liquid crystal is##STR34## and said additional liquid crystal is a second liquid crystalof the formula ##STR35## and a third liquid crystal of the formula##STR36## and wherein the molar proportions of said first, second andthird liquid crystals are 0.15, 0.22 and 0.64, respectively.
 9. Theliquid crystal composition according to claim 6, wherein said firstliquid crystal is ##STR37## and said additional liquid crystal is asecond liquid crystal of the formula ##STR38## a third liquid crystal ofthe formula ##STR39## and a fourth liquid crystal of the formula##STR40## and wherein the molar proportions of said first, second, thirdand fourth liquid crystals are 0.13, 0.54, 0.18 and 0.15, respectively.10. The liquid crystal composition according to claim 6, wherein saidfirst liquid crystal is ##STR41## and said additional liquid crystal isa second liquid crystal of the formula ##STR42## a third liquid crystalof the formula ##STR43## and a fourth liquid crystal of the formula##STR44## and wherein the molar proportions of said first, second, thirdand fourth liquid crystals are 0.14, 0.58, 0.19 and 0.09, respectively.